This invention relates to an apparatus for positioning an object by means of a servo-control system.
Conventionally, in an apparatus for quickly and accurately positioning an object, a digital servo-control system which utilizes a feed back signal or signals based on velocity and position has been used. A typical example of such a known system is illustrated in FIG. 1, wherein the reference number 1 designates a position counter providing an output denoting a direction and a distance to a predetermined stopping point. The position counter 1 reduces its count one by one in response to receipt of a position pulse signal from a position detector 8. The position detector 8, which is mechanically connected to a servo-motor 6, generates a position pulse signal every time a controlled object (not shown in the drawing) moves a unit distance.
A velocity detector 7, which is also connected mechanically to the servo-motor 6, generates a voltage in proportion to the rotating velocity of the motor 6, and supplies this output voltage to an analog adder 4 which makes an addition of the inputs thereto and provides an output analog value. An analog power amplifier 5 connected to the output of the adder 4 provides power amplification of the analog amount.
A circuit 2 for generating a reference velocity receives a signal from the position counter 1 and produces an analog signal of a reference velocity value X.sub.01 corresponding to the position signal X.sub.1 as shown in FIG. 2. The reference velocity value X.sub.01 represents a reference velocity in the velocity servo-loop comprising the power amplifier 5, the servo-motor 6, and the velocity detector 7. In the circuit 2 for determining a reference velocity, the reference velocity value X.sub.0 is reduced, as shown in FIG. 2, as the position X of the object approaches the origin or stopping point.
The apparatus for positioning which includes such an analog velocity control system, controls the velocity pattern at stopping by means of a negative feed back of the analog velocity signal, to thereby position the object quickly, precisely and stably. But the conventional positioning apparatus based on the above-described analog velocity control system has a disadvantage in that the circuit for determining the reference velocity establishes a reference velocity value X.sub.0 with respect to the position X of the object as shown in FIG. 2.
In order to decelerate smoothly, a reference velocity value should not show a graded characteristic, as shown in FIG. 2, but is required to approach a character of a smooth and continuous curve by increasing the number of steps. But an increase in the number of steps in the pattern makes the circuit for determining the reference velocity more complicated and expensive. Generally, when in a servo system of this type, the inertia moment and the load friction torque of a controlled object are changed, the characteristic of the circuit 2 for determining the reference velocity has to be varied according to the change of the controlled object.
In order to change the characteristic of the circuit 2, the whole construction of the circuit 2 is required to be changed. This means that the circuit 2 has to be designed again from the beginning. That is to say, the conventional apparatus for positioning as shown in FIG. 1 has another disadvantage in that it is lacking in adaptability and the degree of freedom of change in characteristic.